FIG. 6 illustrates the structure of a prior-art dry-ice blast device. The prior-art device includes an intermediate/high pressure type air compressor 90 for compressing air to no less than 8 atmospheres (atm). The device also includes an air dryer 91, a pipe 92 and a nozzle 93 which are connected to the discharge port of the air compressor. Further, the device is provided with a hopper 94 and a feeding unit 95 for feeding dry-ice particles D stored in the hopper to a pipe 92a branching from the pipe 92. After fed into the pipe 92a, the dry-ice particles D are put into the air flow in the nozzle 93.
With the above arrangement, the high-speed air flow with dry-ice particles D is discharged through the nozzle 93, so that the required blast treatment is performed by utilizing the dry-ice particles D as abrasives. The spent dry-ice particles D will sublime in a short period of time after hitting on the object of the blast treatment, which eliminates the need to collect the ejected particles.
It has been found, however, that the prior-art device suffers the following drawbacks.
First, the high-speed air jetted out through the nozzle 93 generates deafening noise, the sound pressure level of which may even reach 120 dB. Conceivably, this jet sound is produced when the air rapidly expands due to the sudden pressure change. Conventionally, when the operator performs the blast treatment while holding the nozzle 93 with his or her hand, the annoying noise is to be generated near the operator, which makes continuous work unbearable.
In the second place, when the high-speed air flow is jetted out through the nozzle 93, the air of no less than 8 atm rapidly expands to the atmospheric pressure, thereby generating a turbulent flow inside the nozzle 93. Due to this, the dry-ice particles D may be broken within the nozzle 93, which is a waste of the dry-ice particles.
In the third place, in passing through the pipe 92a, the dry-ice particles D strongly hit against the inner wall surface of the pipe 92a. This also causes the breakage or excessive wearing of the dry-ice particles.
In the fourth place, since the air compressor 90 is of an intermediate or high pressure type, the air compressor 90 and the attached equipment are expensive. Further, as being large and heavy, they are inconvenient for transfer. Conventionally, it is a predominant idea that the efficiency of blast treatment is enhanced as the air compressor generates a higher pressure of air flow. This is the reason why air compressors of an intermediate or high pressure type have been used. However, it will be understood from the following description of the present invention that the idea is not necessarily proper.
In the fifth place, upon reaching the nozzle 93, the dry-ice particles D are jetted out through the nozzle 93 by the high-speed air flow. In this manner, it is difficult to make stable the jetting direction of the dry-ice particles D. In the prior art device, since the pressure change is large when the high-speed air flow jets out through the nozzle 93 to the atmospheric pressure, the air flow is likely to spread through a relatively large angle, causing the dry-ice particles D to spread largely. In light of this, the device is not suitable for exclusive application of the blast treatment to the desired portion.
In the sixth place, since the pipe 92 and the branching pipe 92a are both connected to the nozzle 93, the operator needs to pull around the two pipes together, with the nozzle 93 held in the hands. This can be hindrance to smooth operation.